Improving the efficiency of a transmitter is an important task. Improving the efficiency of a power amplifier associated with the transmitter becomes the first priority of many studies. Much recent research effort has demonstrated the feasibility of integrating most transceiver building blocks into a single die. One of the few remaining blocks is the power amplifier.
Power amplifiers are typically categorized as class-A, class-AB, class-B, class-C, etc., depending upon the bias point. The bias point will affect the efficiency and linearity of a power amplifier. However, within the same class, the efficiency of a power amplifier will mainly be limited by supply voltage, load impedance, and output power.
For example, an ideal resistor load class-A power amplifier may be biased with some DC current. Maximum output voltage will not exceed the supply voltage. Load impedance and supply voltage will define maximum output power. For best efficiency at maximum output, the circuit can be biased and DC output voltage made half of the supply voltage. The load impedance can be chosen to make the maximum output swing exactly the voltage difference between the supply voltage and reference ground.
At small signal, constant biased DC current with small output signal yields low efficiency. Maximum efficiency occurs at maximum output swing; the theoretical maximum efficiency of an ideal resistor load class-A amplifier is 25%. For a larger maximum output signal, the amplifier needs lower load impedance. However, compared to a typical power amplifier at small signal, the amplifier will have lower efficiency because the DC current will be higher.
Similar trade-offs happen with other types of power amplifiers. The load impedance will define the maximum output power and small signal efficiency. Since modern digital communications often have large and small signal components in a signal structure, there are ongoing efforts to develop technologies that improve small signal efficiency without significantly degrading large signal efficiency.
Doherty power amplifiers can be used to try to decouple the output loading impedance, which defines maximum output power, and load impedance at small signal. A normal Doherty power amplifier has a main power amplifier, an auxiliary power amplifier, an impedance inverter at output for signal combining, and a phase shifter before the auxiliary power amplifier to adjust the auxiliary signal phase for proper signal combination.
A normal Doherty power amplifier needs to control the analog gain of 2 paths to achieve the best combining. To overcome device variation, most of these Doherty amplifiers have some mechanisms to control analog gain and block phase. Those controls must be applied at Radio Frequency (RF) blocks. It is difficult to freely decouple the control of gain and phase using known techniques.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.